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There is this service menu where I can select different USB modes for my phone. One option is called CP and the other is AP.

menu

What do the acronyms CP and AP stand for? I recognize MTP, PTP and ADB. But what are the options CP and AP used for?

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  • I should have mentioned that, yes. It's a Galaxy S4 Plus.
    – Samir
    Aug 29, 2015 at 23:44
  • I now understand from the answer below what CP and AP is. What I don't understand is how the selection of either one of these options on the menu affects the operation of the phone. I know, "it enumerates a different driver". So what? To what end? So if someone knows the answer to that please fill it in below.
    – Samir
    Aug 30, 2015 at 9:38
  • The code to get this screen is #*0808# for Android 4.2.2 and above (if I recall correctly). In case someone else wants to try it.
    – Samir
    Aug 31, 2015 at 15:42
  • 3
    @sammyg *#0808#
    – janot
    Nov 1, 2016 at 21:58

1 Answer 1

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TL;DR
Application processor (AP) = CPU + GPU
Cellular processor (CP) = Cellular modem

The long version:
CP stands for Cellular Processor. It's also referred to as Baseband Modem. Another reference is Baseband Processor, or BP for short. AP stands for Application Processor. These are 2 of the 3 fundamental building blocks in mobile devices. The third being the Graphical Processor, or GP for short. The CP includes all the digital components required to communicate with a cellular network. The CP usually consists of an ARM-based processor and a DSP. It has its own operating system and communicates via a HS (High-Speed) serial connection with AP unit on which operating systems such as Android, iOS and Windows Phone are running.

Previously, CP, AP and (GP) were found in separate, dedicated chips. However, there is a trend to combine all three components on a single System on a Chip (SoC) to save cost, reduce power consumption, and shrink the overall size of the circuit board.

The table below is showing SoC chips with two AP — CPU & GPU and one CP — the cellular or baseband processor. The table is copied from 3G, 4G and Beyond by Martin Sauter.

Table1

Bonus:
The USB settings in the image above is used to determine what drivers are enumerated when connecting your phone to the PC via USB cable. It is a multiplexer switch which determines whether the USB port is directly connected to the CP or the AP.

There are also options to select device features that will be enabled once connected to the PC. These options include:

  • MTP (Media Transfer Protocol)
  • ADB (Android Debugging Bridge)
  • PTP (Picture Transfer Protocol)
  • RNDIS (Remote Network Driver Interface Specification) for USB Ethernet
  • DM (Diagnostic Mode)
  • MIDI (Used to connect MIDI device such as MIDI keyboard)
  • Modem

USB connection Options
Click on image to enlarge

More info can be found on XDA forums and in 3G, 4G and Beyond by Martin Sauter.

Reality:
AP mode of UART serial connection is favored upon.
Many blogs seem to have found that AP mode saves battery capacity,
charges phone faster when connected via USB, increases battery life.

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4
  • But what difference does it make if the CP mode or the AP mode is selected? How do these changes manifest? What's the difference? Or when is one preferred over the other?
    – Samir
    Aug 30, 2015 at 9:33
  • Can you please tell me what these CP/AP options do? You have explained the abbreviations and that CP is one component inside the phone, and AP is another. I thank you for that! But what difference does it make which mode is selected? Surely, both CP and AP are enabled in hardware? I tried looking for this info on the XDA forums link but didn't find anything more than a reference to it, and I did a term search in the book mentioned above but got no hits.
    – Samir
    Aug 31, 2015 at 15:39
  • I found one clue about CP/AP, and it's from a discussion at Android Central. One Galaxy S4 user with AT&T in the US had to use this service menu to switch from CP to AP, in order for Samsung Kies software on the PC to recognize his phone. It kept saying SGH-I337 does not support initialising. It seemed to have something to do with a custom bootloader by AT&T, and it was probably set to CP mode by default. That's a clue, but not a clear fact. Switching it to AP made it recognized in Kies.
    – Samir
    Aug 31, 2015 at 15:47
  • It appears to me that this comes down to having the phone behave and operate more closely like a cellular network device (CP), closely tied to its network, or as a mobile computer (AP), with a more independent spirit. It's like comparing a dumb GSM phone of the 1990's on a cellular network with telephone function only, and smart-like PDA devices that resemble mobile devices of today.
    – Samir
    Sep 13, 2015 at 13:23

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